Japanese Laid-Open Patent Application Tokukaisho 63-90221/1988 (published on Apr. 21, 1988) discloses a digitalization circuit using image sensor output, which includes output means for outputting intermittent image signals from an image sensor for scanning data of the document surface, first digitalizing means for digitalizing image signals outputted from the output means based on the integral value of the image signals, second digitalizing means for digitalizing the image signals outputted from the output means based on a single threshold voltage, and output difference detecting means for detecting the difference between two image signals adjacent to each other of the image signals outputted from the output means, wherein when the output difference detected by the detection means exceeds a predetermined value, a digitalization signal of the first digitalizing means is selected; and when the output difference is at or less than the predetermined value, a digitalization signal of the second digitalizing means is selected.